This invention relates to a method of welding and, more particularly, a method of welding together to form a leak-proof and pressure-resistant seal of a member sandwiched between two weldable metallic work pieces.
The method of this invention has particular applicability to the assembly of devices intended to communicate pressure in fluid pressure-operated devices. Such devices are intended to transmit liquid or gas pressure, control the flow of liquids in hydraulic systems, prevent the extensive loss of liquids or gases in ruptured hydraulic or gaseous systems, and transmit pressure of liquids, solids, or gases to such liquid or gas-operated instruments as pressure gauges, indicators, recorders, pressure transmitters, flow valves, or the like. Examples of such devices are shown in U.S. Pat. No. 4,111,056 by the applicant herein; by Green in U.S. Pat. No. 2,841,984; Bissell et al. in U.S. Pat. No. 3,202,063; and the like.
Such devices have in common a housing. The housing has therein a cavity. A diaphragm is within the housing cavity and divides the cavity into two chambers. The diaphragm is secured between sections of the housing. The securing of the diaphragm to the housing is of much concern.
One essential characteristic of such devices is that the diaphragm be held so securely in place that the fluid pressures upon the diaphragm will not break the diaphragm loose from the housing nor cause a rupture or leak in the housing at the juncture of the housing and diapgragm. It is also important that, when the housing and diaphragm are assembled, the diaphragm not be distorted by the joining process itself.
In the past, a number of proposals have been made to secure diaphragms in housings. Such methods of joining generally include welding, braising, or soldering. Thus, for example, Akeley, in U.S. Pat. No. 4,046,010, discloses a method of combining a diaphragm made of tantalum with a stainless steel weld ring and one side of a housing made of stainless steel. The stainless steel weld ring is welded directly to the stainless steel housing at a temperature above the melting point of the stainless steel, but below the melting point of the tantalum, whereby, simultaneous to the welding of the weld ring to the body, a braise is created between the tantalum diaphragm and both the stainless steel weld ring and the stainless steel body. However, the device taught by Akeley does not provide for a pressure-resistant seal, since the joining of the diaphragm to the weld ring and lower housing is the only connection. A braised joining is clearly weaker than welding, which is performed at much higher temperatures. Further, the upper housing of the device taught by Akeley abuts the weld ring and is merely clamped to the lower housing. It should be noted that Akeley requires the use of an elastomeric seal to buffer or keep the materials within the housing chamber from contacting the weld ring. The reason for this is that the weld ring is sensitive to, and may be corroded by, the substances within the chamber. It is clear, of course, that seals are subject to failure and that braising the diaphragm to a weld ring on only one side of a housing does not provide for a strong pressure-tight seal.
Another approach to joining a thin diaphragm to a housing is taught by Canonico et al. in U.S. Pat. No. 3,458,683. Canonico et al. discloses a thin metal foil positioned in a sandwich structure which is in face contact between the bottom metallic work piece and the top metallic retaining member. The sandwich-type structures are secured together by passing electron beams simultaneously through the top retaining member in a metallic foil and bottom work piece. Canonico et al. suggests that the foil member may be made of a material which is metallurgically incompatible with the top retaining member or bottom work piece, and particularly refers to the use of such materials as tantalum and the like. However, the weldment taught by Canonico et al. is perpendicular to the mating area before the foil and the housing. Accordingly, a pressure weld is not formed. In a setting of a pressure transducer or valve, pressure at the edges would become points of failure.
Further, it has been observed that prior consideration of welding techniques has specified that, in order to form a strong metallurgical bond, the mating surfaces must be placed into intimate contact and the weld strength be proportional to the surface areas joined and to the intimacy of contact. However, other characteristics are believed not to have been taken into account in the prior art devices. These observed characteristics, as more fully discussed hereinafter, may be employed to provide a pressure and seal tight weld. The disadvantages heretofore experienced of weak welds and the welding of dissimilar materials is, therefore, overcome.